Method for producing oligosilanes

ABSTRACT

A method for producing oligosilanes by reacting halogenated oligosilanes with a metal hydride includes a reaction occurring in the presence of a catalyst and an alkali metal halide, the catalyst including a halide of a multivalent metal; and the reaction occurs in an ethereal solution.

Related Applications

This is a §371 of International Application No. PCT/EP2010/068995, withan international filing date of Dec. 6, 2010, which is based on GermanPatent Application No. 10 2009 056 731.3, filed Dec. 4, 2009, thesubject matter of which is incorporated by reference.

TECHNICAL FIELD

This disclosure relates to a method for producing oligosilanes.

BACKGROUND

Oligosilanes are usually produced by hydrogenation of halogenatedsilicon compounds with metal or semimetal hydrides. Methods reliant onhigh reaction temperatures, for example, hydrogenation of halogenatedsilicon compounds in the absence of solvent are not very suitable forpreparing oligosilanes. At high reaction temperatures, decomposition ofa silicon compound or of an oligosilane can occur with dissociation of amonosilane, for example. Such dissociation can be effected, for example,as a result of a reductive cleavage of a Si—Si bond. Methods that aresufficiently fast at low reaction temperatures need readily solublecomplex metal hydrides. These complex metal hydrides, for example, metalborohydrides or metal aluminum hydrides, are more soluble than alkali oralkaline earth metal hydrides, but lead to a significant increase inmanufacturing costs.

Methods for producing oligosilanes are therefore desired to be simplerand more economical to carry out and provide oligosilanes in highpurity. It could therefore be helpful to provide an improved and moreeconomical method for producing oligosilanes.

SUMMARY

We provide a method for producing oligosilanes by reacting halogenatedoligosilanes with a metal hydride, wherein the reaction takes place inthe presence of a catalyst and of an alkali metal halide, the catalystcomprises a halide of a multivalent metal, and the reaction takes placein an ethereal solvent.

DETAILED DESCRIPTION

We provide a method for producing oligosilanes. Oligosilanes areproduced according to this method by reacting halogenated oligosilaneswith a metal hydride, wherein

-   -   the reaction takes place in the presence of a catalyst and of an        alkali metal halide;    -   wherein the catalyst comprises a halide of a multivalent metal;        and    -   the reaction takes place in an ethereal solvent.

“Halogenated oligosilanes” are predominantly or completely substitutedwith halogen atoms. “Oligosilanes” are predominantly or completelysubstituted with hydrogen atoms. The halogenated oligosilanes arereduced by our method to oligosilanes. The oligosilanes and halogenatedoligosilanes may each be mixtures of compounds or single compounds.Oligosilanes and halogenated oligosilanes are compounds which eitherhave a single central silicon atom or, if they have two or more siliconatoms, they are interconnected by Si—Si bonds. More particularly,oligosilanes and halogenated oligosilanes have from 1 to 8 siliconatoms.

A multivalent metal has an oxidation number>1. Alkali metals are notmultivalent metals, their oxidation number in halides is=1. Alkali metalhalides herein are not counted as catalysts. A reaction mixture may be asolution and more particularly a suspension and may be through-mixed byshaking or stirring.

The alkali metal halide serves inter alia to improve the solubility ofthe metal hydride and of other reactive species so that their reactivityis enhanced, or to convert the catalyst into a more reactive form. It isthus the case that in the presence of the alkali metal halide,conversion of halogenated oligosilanes into oligosilanes is acceleratedcompared with a conventional method in the absence of such an alkalimetal halide. This makes it possible to use smaller amounts of catalystwhich can act as a Lewis acid. Yet the reaction rate is not adverselyaffected by the low concentration of catalyst and may even be enhanced.Thus, in our method, the amount of catalyst added can be reduced by theless costly alkali metal halide, to thereby lower manufacturing costs.Alkali metal halides are ecologically unconcerning, and so the method isnot just more economical than conventional methods, but also has alesser environmental impact.

Combining catalyst and alkali metal halide makes it possible to performconversion of halogenated oligosilanes to oligosilanes at low reactiontemperatures. As a result, undesired decompositions such as dissociationof monosilanes for example, can be reduced or almost completelyprevented. Almost completely is to be understood as meaning to an extentof at least 95% and more particularly to an extent of at least 99%.Since less decomposition occurs, the overall yield of the methodincreases, making it more economical. Advantageously, the desiredoligosilanes are also obtained in high purity, since they are lesscontaminated with decomposition products than is the case withconventional methods.

Oligosilanes are generally volatile and advantageously removable fromthe reaction mixture via the gas phase and are thus easy to isolate.

The catalyst may be used in superstoichiometric quantity relative to themetal hydride. That is, more than one equivalent of catalyst can be usedper equivalent of metal hydride.

The catalyst may be used in a stoichiometric or a substoichiometricamount relative to the metal hydride. The catalyst may preferably beused in a substoichiometric amount relative to the metal hydride.“Substoichiometric” amount is to be understood as meaning an amount ofless than one equivalent. That is, less than one equivalent of catalystper equivalent of metal hydride can be used in the method.

The metal hydride may be used in a molar ratio to catalyst ranging from1:1 to 200:1. The metal hydride can be used in a molar ratio to catalystranging from 4:1 to 150:1, preferably from 20:1 to 100:1. In particular,the combination of the catalyst with the alkali metal halide makes itpossible to use particularly small amounts of the catalyst in themethod.

The alkali metal halide may be used in a substoichiometric amountrelative to the metal hydride.

The catalyst may be used in a ratio to alkali metal halide of 1:4 to10:1 and preferably 1:3 to 2:1. Examples of the method in which onlysmall amounts of alkali metal halide are needed are particularlyadvantageous. Since both the catalyst and alkali metal halide can beused in substoichiometric amounts, the apparatus requirements of themethod are also reduced. For example, the reaction mixture is easier tothrough-mix in those cases where the catalyst and/or the alkali metalhalide are not present in solution or only present to a small degree.This in turn facilitates the method.

The metal hydride may be used in a molar ratio to halide contained inthe halogenated oligosilane of 1:1.1 to 5:1 and particularly 1:1 to1.5:1. Advantageously, therefore, the method requires onlystoichiometric or slightly superstoichiometric amounts of metal hydrideper halide in the silane, even in the presence of small amounts ofcatalyst. The molar amount of halide in the halogenated oligosilane isgreater than the molar amount of the halogenated oligosilane. Forexample, the molar amount of chloride in the halogenated oligosilaneSi₂Cl₆ is equal to six times the molar amount of Si₂Cl₆ used.

A metal hydride may be used which comprises or consists of an alkalimetal hydride, an alkaline earth metal hydride or a combination thereof.The alkali metal hydride used can be lithium hydride (LiH), sodiumhydride (NaH), potassium hydride (KH) and a combination thereof. Thealkaline earth metal hydride used can be magnesium hydride (MgH₂),calcium hydride (CaH₂) and a combination thereof.

In general, alkali metal hydrides or alkaline earth metal hydrides areless costly than the complex hydrides, for example, metal borohydridesor metal aluminum hydrides obtained therefrom. Therefore, our method ismore economical than conventional methods involving complex metalhydrides since the alkali metal and/or alkaline earth metal hydrides areactivated by adding substoichiometric amounts of catalyst compounds andalkali metal halides. Catalyst compounds are more particularly thecatalysts recited hereinbelow. Examples of complex hydrides are sodiumborohydride, lithium aluminum hydride and Red-Al®.

Advantageously, alkali metal hydrides and alkaline earth metal hydridesare not just less costly than complex hydrides, they are also easier tohandle. Some alkali metal hydrides are commercially available at lowcost in the form of a dispersion in organic solvents and can also beused in our method in the form of that dispersion. The metal hydride insuch dispersions is well protected from moisture and atmospheric oxygen.This provides more particularly a technical advantage over conventionalmethods involving complex metal hydrides, which are very sensitive tomoisture and/or atmospheric oxygen. This also makes the method safersince the risk of upsets due to spontaneous, uncontrolled side reactionsis reduced. A further advantage of alkali metal hydrides and alkalineearth metal hydrides is that, unlike borohydrides or metal borohydrides,they cannot release any volatile toxic compounds such as diborane.

A metal hydride may be used which comprises or consists of an alkalimetal hydride. More particularly, the alkali metal hydride may compriseor consist of lithium hydride. Advantageously, lithium hydride is veryinexpensive and has a low molecular weight. Furthermore, the massfraction of hydride is very much larger in lithium hydride than in othermetal hydrides. Therefore, the mass of metal hydride used can be reducedcompared with other metal hydrides, reducing manufacturing costs andwaste. Lithium hydride is advantageously also easy to handle in the formof the pure solid material.

The reaction may be carried out in an ethereal solvent comprising orconsisting of a first solvent having an ether group and optionally asecond solvent. Nonlimiting examples of first solvents, which contain atleast one ether group, are diethyl ether, tetrahydrofuran, dipropylether, butyl methyl ether, dibutyl ether, diphenyl ether, dioxane,dimethoxyethane or diethylene glycol dimethyl ether. The etherealsolvent may also comprise or consist of a combination of first solvents.Therefore, the ethereal solvent may contain at least one first solventhaving an ether group, also called ether function, or consist thereof. Afirst solvent having an ether function is important for an efficientreaction because it improves solubility and/or is important forstabilizing some species in the reaction mixture.

Nonlimiting examples of the second solvent which does not contain anether group, optionally present in the ethereal solvent are aromaticcompounds such as toluene, xylene, ethylbenzene or alkanes such asoctane, decane or paraffin oil and also mixtures thereof. It is alsopossible for the reaction mixture to contain a mineral oil as a secondsolvent.

In the mentioned first and second solvents, the alkyl substituents alsorepresent branched alkyl substituents. Propyl thus represents bothn-propyl and isopropyl, which means that dipropyl ether, for example, isrepresentative of di-n-propyl ether, n-propyl isopropyl ether anddiisopropyl ether. Similarly, butyl represents each of n-butyl,sec-butyl, tert-butyl and isobutyl. The solvents are further alsorepresentative of the entire family of solvents, i.e., xylene isrepresentative of ortho-, meta- and para-xylene, octane isrepresentative of n-octane or a branched octane, decane isrepresentative of n-decane or a branched decane, or the like.

Solvents used more particularly in the method have a higher boilingpoint and hence a lower vapor pressure than the desired product, theoligosilane. As a result, the solvent does not evaporate as readily asthe desired reaction product, and so the oligosilane is preferentiallyremovable from the reaction mixture via the gas phase. The oligosilaneis thus easy to isolate. An example of a possible reaction product isthe oligosilane Si₃H₈, which has a boiling point of about 60° C. understandard conditions. To produce Si₃H₈ from Si₃Cl₈, therefore, it ispreferable to select a solvent having a boiling point of >60° C.

The catalyst may be a halide of a multivalent metal selected from thegroup consisting of: aluminum fluoride, aluminum chloride, aluminumbromide, aluminum iodide, gallium fluoride, gallium chloride, galliumbromide, gallium iodide and a combination thereof. A catalyst is usedmore particularly, which comprises or consists of aluminum chloride,aluminum bromide and a combination thereof. These inorganic catalystsare generally more economical than boron, aluminum or gallium compoundscomprising organic substituents, wherein these organic substituents maybe alkyl, aryl or alkoxy substituents for example. The inorganiccatalysts used in the method are not just less costly than theseorganically substituted compounds, but they are also easier to handle,since they are not pyrophoric and not so sensitive to moisture and/oratmospheric oxygen. As a result, the above-described advantages arelikewise achieved for our method.

Aluminum chloride may be used as catalyst. We realized that aluminumchloride in conjunction with an alkali metal halide can be used withoutfurther catalysts, which are generally costlier than aluminum chloride.This makes our method more economical than conventional methods. Forexample, using pure aluminum chloride is less costly than using aluminumbromide since the latter is significantly more expensive. It istherefore thus possible for our method to be more particularly lesscostly to carry out than other, conventional methods.

An alkali metal halide may be used, which is selected from the groupconsisting of: lithium chloride, lithium bromide, lithium iodide, sodiumchloride, sodium bromide, sodium iodide and a combination thereof. It ismore particularly possible to use an alkali metal halide selected fromlithium chloride, lithium bromide and a combination thereof. It is moreparticularly possible to use lithium bromide as alkali metal halidesince it is more soluble in ethereal solvents and, hence, is morereactive than lithium chloride. It is particularly advantageous to havea combination of substoichiometric amounts of lithium bromide withsubstoichiometric amounts of aluminum chloride as catalyst for themethod.

Halogenated oligosilanes may have a composition represented by theformula Si_(n)H_(p)X_(m−p). Here n=1 to 8, m=2n to 2n+2, 0≦p<0.5*m andX=F, Cl, Br, I or a combination thereof. More particularly, it ispossible for X=F, Cl or a combination thereof. It is possible for n=1 to5 and more particularly also n=2 to 5. It may be preferable for0≦p<0.1*m and more particularly preferable for 0≦p<0.01*m to apply. Themethod may utilize not only linear, branched but also cyclic halogenatedoligosilanes in order that the corresponding linear, branched as well ascyclic oligosilanes may be produced.

The halogenated oligosilanes are oligochlorosilanes which have acomposition represented by the formula Si_(n)Cl_(m). In this formula,m=2n to 2n+2. More particularly, m=2n+2, so that the oligochlorosilanesare represented by the formula Si_(n)Cl_(2n+2). Here it is possible forn=1 to 8, preferably n=1 to 5 and more preferably n=1 to 3.

Oligosilanes are produced from the halogenated oligosilanes having acomposition represented by the formula Si_(n)H_(q)X_(m−q), where0.95*m≦q≦m and more particularly 0.99*m≦q≦m. It is possible for n and mto be selected as described above.

The reaction may take place at a pressure of 10 hPa to 1500 hPa. Thereaction can take place more particularly at a pressure of 10 hPa to 500hPa. More particularly, the reaction can take place at an underpressure,i.e., at below 1000 hPa, as a result of which the reaction product, theoligosilane, is easy to remove from the reaction mixture and, hence,easy to isolate. An underpressure reaction further makes it possible toremove the reaction product from the reaction mixture at lowtemperatures via the gas phase.

The reaction takes place at a temperature between −20° C. and theboiling point of the solvent. The reaction can also take place at −10°C. to 70° C. and more particularly 0° C. to 40° C.

The method may comprise the steps of:

-   -   (a) adding metal hydride and solvent;    -   (b) adding catalyst and alkali metal halide;    -   (c) adding halogenated oligosilane.

The solvent may have an ether group, i.e., be an ethereal solvent asdescribed above, or a further solvent having an ether group can be addedin some other step, so that altogether an ethereal solvent is used inthe method. The order of the reaction steps mentioned may be anydesired, but this order is adopted in particular. Individual reactionsteps may take place at the same time or be carried out together. Theindividual constituents of the reaction mixture may be chosen inaccordance with the examples described.

The metal hydride can be added in step (a) either in pure form, as asuspension in an ethereal solvent or as a dispersion in an organicsolvent. In step (b), the catalyst and the alkali metal halide can beadded as solids, in dissolved form and/or as a suspension. The twocompounds can be used separately from each other or as mixture. When thetwo compounds are added separately from each other, the order in whichthey are added is freely choosable.

The reaction may take place under mixing of liquid and solid phase, forexample, by shaking or stirring, more particularly by stirring. Mixingcan take place in one or more steps.

The halogenated oligosilane may be added in step (c) by meteredaddition. That is, the oligosilane is more particularly added incontrolled fashion.

The oligosilane formed may be removed from the reaction mixture ingaseous form in a further step (d). Isolating and enriching can takeplace separately. Step (d) can take place simultaneously or partlysimultaneously with one or more other steps, more particularlysimultaneously or partly simultaneously with step (c).

In the following, a further example of the method for the production ofoligosilanes by reacting halogenated oligosilanes with a metal hydrideis described, which combines multiple examples of the method. Thisversion may be more particularly complemented in any desired manner withthe further recited examples.

To produce oligosilanes Si_(n)H₂₊₂ (n=1-3) from respectivelytetrachlorosilane SiCl₄, hexachlorodisilane Si₂Cl₆ oroctachlorotrisilane Si₃Cl₈, alkali metal hydride is initially charged ina solvent which contains at least one ether group or mixtures ofsolvents which each contain at least one ether group or mixtures of atleast one solvent which contains at least one ether group with furthersolvents which contain no ether groups, and AlCl₃ and lithium bromideLiBr are added. The oligochlorosilane Si_(n)Cl₂₊₂ (n=1-3) is meteredinto this mixture and the respective reaction product SiH₄, Si₂H₆ orSi₃H₈ is removed from the reaction space in gaseous form.

Nonlimiting examples of solvents containing at least one ether group arediethyl ether, tetrahydrofuran, dipropyl ether, butyl methyl ether,dibutyl ether, diphenyl ether, dioxane, dimethoxyethane or diethyleneglycol dimethyl ether. Nonlimiting examples of solvents containing noether group are aromatic compounds such as toluene, xylene, ethylbenzeneor alkanes such as octane, decane or paraffin oil and also mixturesthereof. Preference is given to using solvents having a higher boilingpoint than the particular product. Accordingly, Si₃H₈ is produced usingsolvents or solvent mixtures having an atmospheric pressure boilingpoint >60° C.

Examples of alkali metal hydride are LiH, sodium hydride (NaH),potassium hydride (KH) or mixtures thereof. Preference is given to usingLiH.

AlCl₃ and LiBr can be added as solids or in dissolved form. The twocompounds can be used separately from each other or as a mixture. Whenthe two compounds are added separately from each other, the order ofaddition is freely choosable.

The reaction is preferably carried out under mixing of liquid and solidphase, for example, by stirring.

The silanes are produced at temperatures between −20° C. and the boilingpoint of the solvent or solvent mixture used. The reaction temperatureis preferably −10° C. to 70° C. and more preferably 0° C. to 40° C.

The method is carried out at a pressure of 10 hPa to 150 kPa. Theproduction of Si₃H₈ at reduced pressure of 10 hPa to 50 kPa ispreferred.

The molar ratio of AlCl₃ used to LiBr used is 1:4 to 10:1 and preferably1:3 to 2:1.

The methods described here are not limited by the description withreference to the operative examples. On the contrary, this disclosurecomprises every novel feature and also every combination of features,which more particularly includes every combination of features in theappended claims, even if that feature or that combination itself is notexplicitly indicated in the claims or operative examples.

1-15. (canceled)
 16. A method for producing oligosilanes by reactinghalogenated oligosilanes with a metal hydride, wherein the reactiontakes place in the presence of a catalyst and of an alkali metal halide;the catalyst comprises a halide of a multivalent metal; and the reactiontakes place in an ethereal solvent.
 17. The method according to claim16, wherein the catalyst is used in a stoichiometric or asubstoichiometric amount relative to the metal hydride.
 18. The methodaccording to claim 16, wherein the metal hydride is used in a molarratio to the catalyst of 1:1 to 200:1.
 19. The method according to claim16, wherein the alkali metal halide is in a substoichiometric amountrelative to the metal hydride.
 20. The method according to claim 16,wherein the catalyst is in a ratio to the alkali metal halide of 1:4 to10:1.
 21. The method according to claim 16, wherein the metal hydride isin a molar ratio to the halide contained in the halogenated silane of1:1.1 to 5:1.
 22. The method according to claim 16, wherein the metalhydride comprises an alkali metal hydride, an alkaline earth metalhydride or a combination thereof.
 23. The method according to claim 16,wherein the metal hydride comprises an alkali metal hydride.
 24. Themethod according to claim 16, wherein the reaction takes place in anethereal solvent comprising a first solvent having an ether group andoptionally a second solvent; wherein the first solvent is selected fromthe group consisting of diethyl ether, tetrahydrofuran, dipropyl ether,butyl methyl ether, dibutyl ether, diphenyl ether, dioxane,dimethoxyethane, diethylene glycol dimethyl ether and combinationsthereof; and the second solvent is selected from the group consisting oftoluene, xylene, ethylbenzene, octane, decane, paraffin oil andcombinations thereof.
 25. The method according to claim 16, wherein thecatalyst is a halide of a multivalent metal selected from the groupconsisting of aluminum fluoride, aluminum chloride, aluminum bromide,aluminum iodide, gallium fluoride, gallium chloride, gallium bromide,gallium iodide and combinations thereof.
 26. The method according toclaim 16, wherein the catalyst is aluminum chloride.
 27. The methodaccording to claim 16, wherein the alkali metal halide is selected fromthe group consisting of lithium chloride, lithium bromide, lithiumiodide, sodium chloride, sodium bromide, sodium iodide and combinationsthereof.
 28. The method according to claim 16, wherein the halogenatedoligosilanes have a composition represented by formulaSi_(n)H_(p)X_(m−p), and where n=1 to 8, m=2n to 2n+2, 0≦p<0.5*m and X=F,Cl, Br, I or a combination thereof.
 29. The method according to claim16, wherein the reaction takes place at a pressure of 10 hPa to 1500hPa.
 30. The method according to claim 16, wherein the reaction takesplace at a temperature between −20° C. and the boiling point of thesolvent.